1. Field of the Invention
The present invention relates to an electrochemical reaction unit.
2. Description of the Related Art
A known type of a fuel cell for generating electricity by utilizing an electrochemical reaction between hydrogen and oxygen is a solid oxide fuel cell (hereinafter, also called “SOFC”) which has electrolyte layers containing a solid oxide. A fuel cell electricity generation unit (hereinafter, also called an “electricity generation unit”), which is the smallest electricity generation unit of a SOFC, has a unit cell which includes an electrolyte layer and a mutually facing cathode and anode with the electrolyte layer intervening therebetween. Further, electrically conductive current collectors are disposed respectively on a cathode side and on an anode side of the unit cell and adapted to collect electricity generated by the unit cell. The cathode and the corresponding current collector are electrically connected and bonded together by an electrically conductive bonding layer. The bonding layer is a porous layer having a plurality of pores formed therein.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2011-99159
3. Problems to be Solved by the Invention
In the conventional structure mentioned above, due to, for example, a difference in the coefficient of thermal expansion between members such as the cathode and the current collectors, stress (e.g., tensile stress) is generated in the bonding layer. As a result, a crack may generate, progressing from the outer surface of the bonding layer to the interior of the bonding layer. In the case where the bonding layer is a porous layer as in the above structure, since the generated stress can be diffused by means of pores, a crack initiating in the outer surface can be prevented from progressing to the interior of the bonding layer. Further, the greater the average diameter of pores in the bonding layer, the higher the stress dispersibility provided by the pores, and thus the progress of a crack can be restrained more reliably.
However, as described below, increasing the diameters of pores in the bonding layer for restraining the progress of a crack may deteriorate electrical conductivity between the current collector and the cathode. For example, in the vicinity of an interface between the bonding layer and the cathode, a plurality of contact portions where the bonding layer and the cathode are in contact with each other line up along the interface with a pore intervening therebetween. The greater the average diameter of pores in the bonding layer, the wider the widths of the contact portions along the interface, and the wider the intervals between the contact portions become. As a result, since current which flows per unit area of each contact portion increases, Joule heat generated per unit area of each contact portion increases. Also, even in the case where one pore having a relatively large diameter exists in the vicinity of the interface between the bonding layer and the cathode, since current which flows per unit area of a contact portion located in the periphery of the pore increases, Joule heat generated per unit area of the contact portion increases. That is, if the average diameter of pores in the bonding layer is large or if a pore having a relatively large diameter exists locally, current concentration will occur in which current flows between the bonding layer and the cathode in a locally concentrated manner to thereby generate a high Joule heat and thus thermally deteriorate the current collector, the cathode, etc. As a result, electrical conductivity between the current collector and the cathode may deteriorate. Such current concentration stemming from the diameters of pores could occur in the vicinity of the interface between the bonding layer and the current collector.
Such a problem is common with an electrolysis cell unit, which is the smallest unit of a solid oxide electrolysis cell (hereinafter, also called “SOEC”) for generating hydrogen by electrolysis of water. As used herein, an electricity generation unit and an electrolysis cell unit are collectively called an electrochemical reaction unit.